Characterization and Integrity Assessment of Friction-Welded Joints Between ETP-Cu and SS304L for Fusion Applications

This study investigates the friction welding of an asymmetrical dissimilar metal electrolytic tough pitch copper (ETP-Cu) plate (100 × 100 × 10 mm) and a Type 304L stainless steel alloy (SS304L) rod (16-mm diameter × 58-mm length) through comprehensive nondestructive and destructive testing techniques to assess and validate the joint quality. Nondestructive tests included visual inspection, in situ hardness measurements, ultrasonic C-scan, mass spectrometer leak detection, high-pressure testing, and vacuum baking cycles. Destructive testing involved tensile testing, fractography analysis, optical and scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analysis. The ultrasonic C-scan provided insights into penetration depth and localized de-bonding. High-pressure testing was conducted at 17 bars using nitrogen gas, with a 30-min holding period, and vacuum baking was performed in four cycles at 180°C for 1 h each. Mass spectrometer leak testing and high-pressure assessments were conducted before and after vacuum baking to evaluate joint integrity. Post-weld measurements confirmed appropriate penetration depth and alignment of the SS304L rod relative to the ETP-Cu plate. Mechanical testing yielded an average tensile strength of 143 MPa, corresponding to 77.6% joint efficiency relative to the ETP-Cu base metal. Microstructural analysis revealed significant changes on the ETP-Cu side, while minimal alterations were observed on the SS304L side. Partial de-bonding was detected along the periphery of the SS304L rod, with widths ranging from 74 to 116 µm. Strong metallurgical bonding was observed at the bottom interface. Elemental diffusion at the interface led to the formation of intermetallic compounds (FeCu₄ and Cu₉Si), as confirmed by EDS and XRD results. Overall, the structural and metallurgical soundness of the friction-welded joints was confirmed, establishing their suitability for use in the ion extractor grid assembly of the neutral beam injection system for fusion applications.